The cmk-pnp double mutants will be studied to assure that they have continued low pools at 42 degrees C. We will determine whether they die at the restrictive temperature. The structural gene encoding nucleoside diphosphokinase will be mapped. Mutants recently isolated in the gene encoding guanosine monophosphokinase will be characterized and mapped. Using fusions between pyrB and lacZ derepressed mutants for pyrB will be isolated as those that produce high levels of beta-galactosidase when grown in the presence of uracil.